In today's multi-host, multi-processor computing environment, the number of processors available to initiate input-output (“I/O”) operations and to perform further processing, when an operation completes, continues to increase. Further flexibility is provided by I/O devices that may incorporate “channel capability” that allows device behavior to be tailored for each operation. In some implementations, the number of channels supported by a device may number in the millions.
For example, a system may be implemented according to the InfiniBand™ Architecture Specification developed by the InfiniBand Trade AssociationSM, the specification for which is incorporated herein by reference (InfiniBand™ Architecture Specification, version 1.0). The InfiniBand™ Architecture defines a system area network for connecting host processor nodes. The term “node” as used in this specification and in any appended claims means an entity in a computer network or system that includes at least one processor but may include arbitrarily many processors. The InfiniBand™ architecture further defines a switched communications fabric allowing many devices to concurrently communicate with high bandwidth and low latency in a protected, remotely managed environment. System nodes can range from a small server with one processor and a few I/O devices to a massively parallel supercomputer installation with hundreds of processors and thousands of I/O devices.
Communication among InfiniBand™ nodes is accomplished according to an InfiniBand™ protocol. In addition, the IP (Internet protocol) friendly nature of the architecture allows bridging to an Internet, Intranet, or connection to remote computer systems. As shown in FIG. 1, system 100 consists of processor nodes 102, 103, and 104 and I/O subsystems 105, 106, 107, and 108 connected through the fabric 101. The fabric is made up of cascaded switches 109 and routers 110. I/O subsystems can range in complexity from a single attached device, such as a SCSI or LAN adapter to large memory rich RAID subsystems 107. The number of input/output channels that can be supported by an intelligent I/O subsystem, such as a Host Channel Adapter (“HCA”), may number in the millions according to the InfiniBand™ specification.
An operation is initiated by a process requesting an I/O channel to perform an operation, e.g., for InfiniBand, sending or receiving a message or performing a direct memory access transfer. Upon completion of an operation, an I/O device may interrupt a processor to signal the need for further processing. If a processor that is targeted for such an interrupt cannot service interrupts (e.g., it has been taken off-line or become disabled), a variety of negative consequences can ensue including fatal timeouts that may crash a computer system. Thus, it may be necessary for the operating system to retarget an interrupt from one processor to another processor for an operation that is in process. Further, it may be desirable to retarget interrupts directed to a given processor to another processor for load balancing purposes, or otherwise. Additionally, it may be useful in some systems to target interrupts for an operation to the processor or node that initiated the operation.